[SRU][Trusty][PATCH 2/2] ipv6: correctly add local routes when lo goes up

[Joseph Salisbury]

From: Nicolas Dichtel <nicolas.dichtel at 6wind.com>

BugLink: http://bugs.launchpad.net/bugs/1634545

The goal of the patch is to fix this scenario:
 ip link add dummy1 type dummy
 ip link set dummy1 up
 ip link set lo down ; ip link set lo up

After that sequence, the local route to the link layer address of dummy1 is
not there anymore.

When the loopback is set down, all local routes are deleted by
addrconf_ifdown()/rt6_ifdown(). At this time, the rt6_info entry still
exists, because the corresponding idev has a reference on it. After the rcu
grace period, dst_rcu_free() is called, and thus ___dst_free(), which will
set obsolete to DST_OBSOLETE_DEAD.

In this case, init_loopback() is called before dst_rcu_free(), thus
obsolete is still sets to something <= 0. So, the function doesn't add the
route again. To avoid that race, let's check the rt6 refcnt instead.

Fixes: 25fb6ca4ed9c ("net IPv6 : Fix broken IPv6 routing table after loopback down-up")
Fixes: a881ae1f625c ("ipv6: don't call addrconf_dst_alloc again when enable lo")
Fixes: 33d99113b110 ("ipv6: reallocate addrconf router for ipv6 address when lo device up")
Reported-by: Francesco Santoro <francesco.santoro at 6wind.com>
Reported-by: Samuel Gauthier <samuel.gauthier at 6wind.com>
CC: Balakumaran Kannan <Balakumaran.Kannan at ap.sony.com>
CC: Maruthi Thotad <Maruthi.Thotad at ap.sony.com>
CC: Sabrina Dubroca <sd at queasysnail.net>
CC: Hannes Frederic Sowa <hannes at stressinduktion.org>
CC: Weilong Chen <chenweilong at huawei.com>
CC: Gao feng <gaofeng at cn.fujitsu.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel at 6wind.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit a220445f9f4382c36a53d8ef3e08165fa27f7e2c)
Signed-off-by: Joseph Salisbury <joseph.salisbury at canonical.com>
---
 net/ipv6/addrconf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 4aabb5f..00191a1 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -2624,7 +2624,7 @@ static void init_loopback(struct net_device *dev)
 				 * lo device down, release this obsolete dst and
 				 * reallocate a new router for ifa.
 				 */
-				if (sp_ifa->rt->dst.obsolete > 0) {
+				if (!atomic_read(&sp_ifa->rt->rt6i_ref)) {
 					ip6_rt_put(sp_ifa->rt);
 					sp_ifa->rt = NULL;
 				} else {
-- 
2.7.4